An important quality requirement of paper and board qualities is the homogeneity of the structure both on a micro scale and macro scale. The structure of the paper produced by the press section must also be symmetric, particularly for paper used in printing applications. Advantageous printing qualities required in printing paper indicate good smoothness, evenness, and certain absorption properties of both faces of the paper.
The properties of paper produced in a paper machine, in particular the symmetry of density, are affected to a considerable extent by the operation of the press section of the paper machine. The symmetric density also has substantial significance with respect to the evenness of the transverse profiles of the paper and the profiles of the paper in the machine direction.
Increased running speeds of modern paper machines create new problems to be solved, most of which relate to the running quality of the machine. At the present time, running speeds of up to about 1400 m/min are used in these modern paper machines. At these running speeds, so-called closed press sections, which typically comprise a compact combination of press rolls arranged around a smooth-faced center roll, usually operate satisfactorily. Examples of such press sections include the applicant's Sym-Press II.TM. and Sym-Press O.TM. press sections.
A particular area of papermaking technology that requires development, in order that the problems associated with the increased running speeds be alleviated, is the center roll of the compact press sections and its material which has commonly been rock. Since rock is a natural material, center rolls made of rock have certain drawbacks such as a tendency to crack caused in part by the nonhomogeneous structure of a rock roll.
In a press section, dewatering a web in a paper machine by means of pressing is preferable to dewatering by evaporation and is economical in terms of energy consumption. For this reason, it is advantageous to remove a maximum proportion of water out of a paper web by pressing in order that the proportion of water that must be removed by evaporation can be made as low as possible. However, the increased running speeds of paper machines provide new, and as yet unsolved, problems expressly in the dewatering of the web by the pressing method. For instance, the press impulse provided in the pressing method cannot be increased sufficiently by the means known in prior art. Furthermore, at high running speeds of the paper machine, the nip times remain unduly short so that the peak compression pressure cannot be increased beyond a certain limit without destroying the structure of the web.
When running speeds of paper machines are increased, the problems of running quality of paper machines are also manifested with increased emphasis because a watery web of low strength cannot withstand an excessively high and sudden impulse of compression pressure or the dynamic forces produced by high speeds. Moreover, web breaks and other disturbance in the operation of the paper machine are produced with resulting standstills. With a modern printing paper machine, the cost of a break standstill is at present about 40,000 FIM, about $8,000, per hour.
Further drawbacks of prior art press sections include the requirement of providing suction energy in the suction rolls as well as the noise problems arising from the suction rolls. Also, suction rolls with perforated mantles, interior suction boxes, and other suction systems are expensive components and require repeated maintenance and servicing.
Additional problems which are manifested with greater frequency at high running speeds of paper machines, and for which a satisfactory solution has not yet been found, include the quality problems related to the requirements of evenness of the longitudinal and transverse property profiles of the paper web. The evenness of the web that is produced in the press section also affects the running quality of the whole paper machine. The evenness of the web is also an important quality factor of finished paper, which is important in respect of copying and printing papers where the requirements of the speeds of copying and printing machines, and uniformity of the printing result, are increased.
The property profiles of the paper produced in the machine direction are also significantly affected by oscillations of the press section and the transverse variations of properties by the transverse profiles of the nip pressures in the press nips. With increasing running speeds of the paper making machine, these profile problems tend to be remarkably increased.
With respect to the prior art related to the present invention, reference is made to Finnish Patent Application Nos. FI 842114 (corresponding to U.S. Pat. No. 4,976,821), FI 842115 (corresponding to U.S. Pat. No. 4,931,143), FI 850627 (corresponding to U.S. Pat. No. 4,561,939), FI 875715 (corresponding to WO 87/06634) and FI 905798, to published Finnish Patent Nos. FI 78,941 (corresponding to U.S. Pat. No. 4,976,820) and FI 80,094 (corresponding to U.S. Pat. No. 4,483,745), and to European Patent No. EP 0 267 186. An object of the present invention is further development and improvement of the prior art press section known from the publications mentioned above.
In published Finnish Patent Application No. FI 909798 (U.S. patent application Ser. No. 07/795,043 now U.S. Pat. No. 5,389,205), a method is described which comprises a combination of the following steps: transferring a paper web from a forming wire onto a wire in a drying section while constantly supporting the web by means of a fabric that receives water, a transfer fabric, or another corresponding transfer surface as a closed draw, preferably at a speed that is higher than about 25 to about 30 m/s; dewatering the paper web by means of at least two subsequent press nips, at least one of which is a so-called extended-nip zone whose length in the machine direction is larger than z&gt;about 100 mm, and forming the extended-nip zone in connection with a mobile flexible press-band loop; and regulating the distribution of the compression pressure employed within the extended-nip press zone both in the transverse direction of the web and in the machine direction so as to set or control the different profiles of properties of the web.
It is an important feature of the method and the device of the above mentioned Finnish Patent Application No. FI 905798 that the paper web is not passed through the press section on one press fabric, but, to guarantee an adequate dewatering capacity, an arrangement of fabrics is employed in which the web is transferred from the pick-up point on the first upper fabric to the drying wire on several fabrics. First, the web is transferred in the first press zone from the first upper fabric to a first lower fabric which runs through the first press zone. The first press zone is preferably an extended-nip zone. The web is thereafter transferred from the first lower fabric onto a second upper fabric which carries the web into a second nip zone. The second nip zone consists of a roll nip, or preferably an extended-nip zone. In the second nip zone, the web is transferred onto a second lower fabric which runs through the second nip zone and carries the web on its upper face as a closed draw onto the drying wire or into an additional nip zone.
The principal dewatering methods in known wire sections described in the above-mentioned documents utilize common dewatering devices, e.g., centrifugal force, foil phenomena, pressing between two wires against a curved shoe, etc. A dry solids content in the range of about 12% to about 20% can be achieved by these methods whereby the pressure applied to the web in the "dewatering points" is fractions of 1 bar. After the wire section, the web still contains plenty of water which is readily removed by methods that are more efficient than those mentioned above. Of the methods mentioned above, the only method based on mechanical pressing is the use of a curved shoe in combination with the tension of a wire in which case the pressure level in a nip is in the order of less than 0.1 bar. However, it is a disadvantage that if such a dewatering step were to be followed by a high-pressure dewatering method, such as an extended nip, there would be a large leap in the pressure applied to dewater the web.